Real-time measurement of tool forces and machining process model parameters

ABSTRACT

A system and method are provided for real-time measurement of tool forces. A relationship between a motor characteristic and tool forces is determined by first directly measuring tool forces for a standard tool, work piece, and part program in a central facility. A force profile indicative of the tool forces and the standard tool, work piece, and part program are provided to a user at a user facility. The motor characteristic for a CNC machine at the user facility is then measured for the standard tool, work piece, and part program. Based on the force profile determined at the central facility, the relationship between the motor characteristic and tool forces is determined. Thereafter, the motor characteristic of the CNC machine is measured for a desired tool, work piece, and part program and converted to tool forces using the relationship between the motor characteristic and tool forces.

RELATED APPLICATIONS

The present application claims priority from provisional U.S. patentapplication Ser. No. 60/534,995, entitled DEVICE FOR ACTIVELY MONITORINGTHE CONDITION OF A TOOL IN A CNC MACHINE, filed Jan. 9, 2004 andprovisional U.S. patent application Ser. No. 60/537,561, entitled METHODFOR REAL-TIME MEASUREMENT OF TOOL FORCES AND MACHINING PROCESS MODELPARAMETERS, filed Jan. 21, 2004, both of which are hereby incorporatedherein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to Computer Numerical Control (CNC)machining, and more particularly relates to a method for real-timemeasurement of tool forces and machining process model parameters.

BACKGROUND OF THE INVENTION

Tool forces depend on the particular tool, cutting geometry, stock (workpiece), and other cutting conditions including whether or not coolant isused. Current methods for measuring tool forces require the use of acomplex and expensive dedicated device such as a piezoelectric forcesensor. Due to the complexity and expense of the force sensor, thesemethods of tool force measurement are most typically used in academicand laboratory studies.

In lieu of direct measurements, tool forces may be predicted forspecified cutting conditions using a suitable process model and processmodel parameters, such as cutting energies. However, the prediction oftool forces using cutting energies does not eliminate the need for toolforce measurements. The process model parameters are obtained bymeasuring tool forces under controlled circumstances and then performinga best fit of these process model parameters to a tool force model. Thismethod provides the tool forces under a variety of cutting geometries,but is limited to the particular combination of tooling, stock, andother cutting conditions used to determine the process model parameters.The predictions are only useful if they may be extended to cuttingconditions beyond those used to determine the cutting energy. However,this extension is prone to substantial errors.

The cutting energies may deviate from their nominal values for a varietyof reasons. A common tool type involves placing an insert into a solidtool body, with the insert forming the cutting edge. While the tool usedto determine the cutting energy may be nominally the same as that usedin practice, minor variations in this insertion process can change theangle of the cutting edge. This changes the effectiveness of theparticular tool, its cutting properties, and the resulting cuttingenergy. Another type of deviation results from the variation in thenominal properties of part materials from job to job. A particularlysevere and important example of this occurs when the initial stock is acasting, where, due to the casting process, the as-cast materialproperties can be quite diverse.

Some tabular data of cutting energies is available for a wide range oftool and part material combinations. As with any laboratorymeasurements, there can be substantial deviations in the actual cuttingenergies from these tabulated values even for nominally the sametabulated tool and part material.

The variation in the cutting energies makes their application to toolforce prediction problematic at best. Further difficulties arise whenthe cutting energies found in one laboratory are transferred to otherapplications. These difficulties are not usually discussed in theresearch literature, since such concerns are often counter to theinterests of the researcher.

In addition, the cutting energies in the tables and in the researchliterature are determined for an ideal (sharp) tool. As the tool wears,the model parameters can change by as much as a factor of two or three,so precision in determining the initial model parameters may not behelpful as the machining process continues.

Thus, cutting energies can only be reliably applied to tool forceprediction when the cutting energies are measured for the particulartool, part, and cutting process under consideration. Applications of thecutting energy values to other conditions may serve as a generalguideline to expected values, but are not expected to be sufficientlyaccurate for applications such as tool condition monitoring and(Numerical Control) NC optimization.

However, in order to measure the cutting energies for a particular tool,part, and cutting process under consideration, tool forces must bedetermined. Since tool forces are traditionally determined usingexpensive and complex dedicated equipment to directly measure toolforces, obtaining cutting energies for tool force prediction for eachindividual job has not been an option. Thus, there remains a need for aninexpensive method for real-time measurement of tool forces and processmodel parameters.

SUMMARY OF THE INVENTION

The present invention provides a system and method for real-timemeasurement of tool forces. In general, a motor characteristic of aComputer Numerical Control (CNC) machine is measured and converted totool forces using a previously determined relationship between the motorcharacteristic and tool forces. The relationship between the motorcharacteristic and tool forces is determined by first directly measuringtool forces for a standard tool, work piece, and part program in acentral facility. The standard tool, work piece, and part program areprovided to a user at a user facility. The motor characteristic, such asmotor power, for a CNC machine at the user facility is then measured forthe standard tool, work piece, and part program. Based on the forceprofile determined at the central facility and the measured motorcharacteristic at the user facility, the relationship between the motorcharacteristic and tool forces for the machine at the user facility isdetermined. Thereafter, the motor characteristic of the CNC machine ismeasured for a desired tool, work piece, and part program and convertedto tool forces for the desired tool, work piece, and part program usingthe relationship between the motor characteristic and tool forces.

Using a process model, a geometric model, and the tool forces for thedesired tool, work piece, and part program, process model parameters,such as cutting energies, may then be determined. The process model andprocess model parameters along with a geometric model for any desiredcutting process may then be used to predict tool forces. The predictedtool forces may be used to perform Numerical Control (NC) optimizationand/or Tool Condition Monitoring (TCM).

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 illustrates an exemplary system for directly measuring toolforces using force sensors in a central facility according to oneembodiment of the present invention;

FIGS. 2A-2B illustrate exemplary embodiments of a system for real-timemeasurement of tool forces and process model parameters using a powersensor according to one embodiment of the present invention; and

FIG. 3 illustrates a method for real-time measurement of tool forces andprocess model parameters according to one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

The present invention provides a system for measuring tool forces andprocess model parameters in real-time without the use of force sensorsthat directly sense tool forces. The force sensors are expensive and,due to their compliance, make the Computer Numerical Control (CNC)machine less stiff and degrade its performance. Indirect sensors, suchas power sensors, are inexpensive and non-invasive. However, thedifficulty is in calibrating the power sensor data, such as spindle orfeed drive power, with regard to tool force. Thus, the present inventionprovides a means for calibrating the power sensor data with respect totool forces. As a result, power sensor data may be measured in real-timeand used to provide real-time measurements of tool forces and processmodel parameters.

FIG. 1 illustrates a system 10 for directly measuring tool forces usinga force sensor 12. In general, the system 10 includes the force sensor12, a Computer Numerical Control (CNC) control system 14, a CNC machine16, a tool 18, a work piece 20, and an analysis system 22. As discussedbelow, the tool 18 and work piece 20 are provided to an end user tocalibrate a CNC machine (FIGS. 2A and 2B) of the end user. Thus, thetool 18 and work piece 20 that are provided to the end user are referredto as a “standard” tool and work piece.

In operation, the CNC control system 14 controls the operation of theCNC machine 16 using a series of commands defined by a part program.More specifically, based on the part program, the CNC control system 14commands motors within the CNC machine 16 to drive and guide the tool 18as the tool 18 cuts the work piece 20.

The work piece 20 is coupled to the force sensor 12 for example usingbolt holes 24-30 with bolts that screw into mating holes 32-38 in theforce sensor 12. Alternatively, the force sensor 12 may be attached tothe work piece 20 using clamps. The force sensor 12 is a multi-axisforce sensor that simultaneously measures force components in the X, Y,and Z directions. In one embodiment, the force sensor 12 is apiezoelectric device that converts the sensed force on the force sensor12 to an electrical force signal. As the tool 18 induces forces in thework piece 20, the forces in the work piece 20 induce forces on theforce sensor 12. The output of the force sensor 12 is provided to theanalysis system 22, where it is amplified, recorded, and analyzed toprovide a force profile and process model parameters.

According to the present invention, a standard part program is providedto the CNC control system 14 for a set of relatively simple cuts on thestandard work piece 20. While the standard part program may berelatively simple, it is sufficiently robust to provide the tool forcedata necessary to compute the process model parameters. The processmodel parameters are the parameters of a process model that predicts thephysical result of a cutting event. More specifically, the process modelis a set of mathematical equations that predicts the system behaviorbased on a geometric model of the cutting event and process modelparameters for a specific tool-material combination. The predictions mayinclude tool forces and torques based on motor characteristics such asCNC spindle and feed drive motor power, current, or torque, as well asaudio, acoustic, ultrasonic and vibration signals. The geometric modelis a solid model representation of the in-process (as-cut) part as wellas the geometry of the material removed from the part in any given toolmove.

While performing the cutting process for the standard part program, theforce sensor 12 measures the tool forces. The output of the force sensor12 is provided to the analysis system 22. Using the sensed forces, ageometric model for the part program (also referred to as a virtualCNC), and a process model, the analysis system 22 generates a forceprofile for the standard part program and calculates the process modelparameters. As described below in detail, the process model parametersfor the preferred process model are cutting energies. More specifically,the force sensor 12 directly measures the tool forces, and the analysissystem 22 performs a best fit process to determine the process modelparameters or cutting energies that relate the geometric model of thecutting process to the tool forces.

Thereafter, the standard tool 18, work piece 20, and part program areprovided to an end user at a user facility. In one embodiment, thestandard tool 18 and work piece 20 are provided to the end user alongwith a computer readable medium containing the standard part program,the force profile, and software for calibrating the user's CNC machine(FIGS. 2A and 2B). It should be noted that the end user does not requiredirect access to the force profile.

Referring to FIGS. 2A and 2B, the user facility includes a system 40 forreal-time measurement of tool forces and process model parametersaccording to one embodiment of the present invention. As describedherein, the end user is at a user facility. It should be noted that theuser facility where the system 40 is located and the central facilitywhere the system 10 of FIG. 1 is located may be at the same geographiclocation or at different geographic locations. In either case, thesystem 10 of FIG. 1 is separate and distinct from the system 40 of FIGS.2A and 2B. More specifically, the CNC machine 16 (FIG. 1) is separateand distinct from the CNC machine 48.

As illustrated in FIG. 2A, the system 40 includes the standard tool 18and work piece 20 from the system 10 (FIG. 1) at the central facility.It should be noted that the system 40 includes the standard tool 18 andwork piece 20 for calibration. Thereafter, the system 40 may include anytool and work piece desired by the end user. The standard part programused to cut the work piece 20 at the central facility, the standard tool18, and the standard work piece 20 are provided to the user facility,which includes a control system 42. The control system 42 includes a CNCcontrol system 44 and a tool force measurement and calibration system46. In one embodiment, the control system 42 is a computer, and the CNCcontrol system 44 and the tool force measurement and calibration system46 are software residing within the control system 42.

In one embodiment, the tool force measurement and calibration system 46is provided to the end user along with the standard part program and theforce profile from the central facility on a computer readable media.The end user then loads the computer readable media into the controlsystem 42 such that the tool force measurement and calibration system 46including the standard part program and the force profile is installedon the control system 42. Thus, the force profile may be incorporatedinto the tool force measurement and calibration system 46 so that theend user does not require direct access or understanding of thisprofile. The end user receives only the standard tool 18, the standardwork piece 20, and the computer readable media from the centralfacility.

Using the standard part program, the control system 42, and specificallythe CNC control system 44, instructs the CNC machine 48 at the userfacility to perform the same set of cuts using the standard tool 18 andwork piece 20 from the central facility. A power sensor 50 senses motorpower on the spindle and/or the feed drives of the CNC machine 48 andprovides a power signal to an amplifier 52. Unlike the force sensor 12(FIG. 1), the power sensor 50 is a relatively inexpensive andnon-invasive power sensor. Further, the power sensor 50 is relativelyeasy to install. In one embodiment, the power sensor 50 may be installedby threading power cables providing power to the spindle and/or feedmotors through holes in the power sensor 50. The amplifier 52 amplifiesthe power signal and provides an amplified power signal to the controlsystem 42, and particularly the tool force measurement and calibrationsystem 46.

Since the standard tool 18 and work piece 20 are the same as used in thecentral facility, the cutting energy parameters (process modelparameters) are also the same. This means that the tool force profileover the standard part program remains the same. The material of thestandard work piece 20 is preferably chosen such that there isnegligible tool wear between the central facility and the user facility.In one embodiment, the standard work piece 20 is an aluminum standardpart. The standard tool 18 may be sited in a shrink-fit tool holder tominimize eccentricity effects (runout). The cutting tests, both at thecentral and at the user facility, may be performed without coolant andlubricants to avoid variations between facilities due to coolant orlubricant type and flow.

Based on the amplified power signal and the force profile obtained atthe central facility, the tool force measurement and calibration system46 determines a relationship between motor power and tool force for theCNC machine 48. The relationship is preferably a transfer functionrelating motor power and tool force for the CNC machine 48. Morespecifically, the spindle and/or feed motor power are measured by thepower sensor 50 over the standard part program. Since the force profileis also known, a match or transfer function is determined that relatesthe motor power to tool forces for the CNC machine 48 at the userfacility. In simplest case, the transfer function is a proportionalityfactor between the spindle power and the tangential force componentand/or between the feed power and the radial force component. Inpractice, some variation in these factors is expected with motor load,spindle speed, feed rate and condition of the motors, which is handledby repeated calibration tests by the end user to determine thedependence of the transfer function on these conditions.

In principle, only one set of tool cuts is needed to determine thetransfer function between motor power and tool force. However, thistransfer function may change with the motor characteristics. As such,the process of determining the transfer function should be repeated forcool (startup) and warm motor conditions, and then repeated over time toaccount for changes, or wear, in motor or belt condition and the likewithin the CNC machine 48. These latter repetitions may be at infrequentintervals, such as, but not limited to, quarterly.

After determining the transfer function relating motor power to toolforces, the standard tool 18, work piece 20, and part program may bereplaced with any desired tool, work piece, and part program. Then, toolforces for the desired tool, work piece, and part program may bemeasured in real-time by measuring motor power and converting themeasured motor power into measured tool forces using the transferfunction relating motor power to tool forces for the CNC machine 48.Accordingly, the tool forces are determined in the user facility withoutthe use of expensive and complex instrumentation.

Further, as described below in more detail, the tool forces may then beused to determine the process model parameters, or cutting energies,which may then be used along with geometric models to predict toolforces for any cutting process performed by the CNC machine 48 with thedesired tool and work piece. These predictions may be used forapplications such as NC optimization and tool condition monitoring.

Referring to FIG. 2B, the illustrated embodiment of the system 40operates essentially the same as the embodiment of FIG. 2A. Thus, forconciseness, the details given above with respect to FIG. 2A will not berepeated. However, in this embodiment, the CNC control system 44operates according to an open source protocol. Thus, the tool forcemeasurement and calibration system 46 may obtain power datarepresentative of the spindle and/or feed power from the CNC controlsystem 44 directly using a software only solution that reads the powerdata from the CNC control system 44 with the assistance of an openarchitecture control. As a result, the cost of the system 40 is furtherreduced by avoiding the use of the power sensor 50 (FIG. 2A).

FIG. 3 illustrates a method 300 for measuring tool forces and processmodel parameters in real-time according to one embodiment of the presentinvention. The process begins by determining a force profile for astandard tool, work piece, and part program at the central facilityusing a force sensor that directly measures tool forces (step 302). Morespecifically, a set of relatively simple cuts defined by the standardpart program are made in the standard work piece using the standardtool. While the standard part program may be relatively simple, thecutting process is sufficiently robust to provide the tool force datanecessary to compute process model parameters, specifically, the cuttingenergies for the standard tool and work piece. Conventional testsdirectly measure the tool forces using a force sensor to generate theforce profile, and a best fit process is used to determine the processmodel parameters or cutting energies that relate the geometric model ofthe cutting process to tool forces.

The standard tool, work piece, and part program are provided to a userfacility from the central facility (step 304). In addition, a computerreadable media is provided to the user facility, as discussed above,wherein the computer readable media stores the tool force measurementand calibration system 46. The standard part program along with theforce profile determined at the central facility for the standard tool,work piece, and part program may be incorporated into the tool forcemeasurement and calibration system 46 software stored on the computerreadable media. As such, the force profile and the standard part programmay be invisible to the end user.

Next, the motor power of a CNC machine at the user facility is measuredfor the standard tool, work piece, and part program (step 306). Itshould be noted that the standard tool, work piece, and part program arethe same as used to determine the force profile at the central facilityin step 302. A transfer function relating the measured motor power tothe force profile determined at the central facility is then determinedat the user facility (step 308).

More specifically, the user replicates the same cuts on the CNC machineat the user facility with the standard tool, work piece, and partprogram. Since the tool and work piece are the same as used in thecentral facility in step 302, the cutting energies are also the same.This means the tool force profile over the part program remains thesame. Since the force profile is known, the spindle and/or feed motorpower are measured over the standard part program, and a match ortransfer function is determined that relates the motor power of the CNCmachine at the user facility to tool forces. In the simplest case, thisis just a proportionality factor between the spindle power and thetangential force component and/or between the feed power and the radialforce component. In practice, some variation in these factors isexpected with motor load, spindle speed, feed rate and condition of themotors, which is handled by repeated calibration tests by the end userto determine the dependence of the transfer function on theseconditions.

In principle, only one set of standard tool cuts is needed to determinethe transfer function between motor power and tool force. However, thistransfer function may change with the motor characteristics and soshould be repeated for a cool (startup) and warm motor and then repeatedover time to account for changes (wear) in motor or belt condition andthe like. These latter repetitions are at infrequent intervals (e.g.quarterly).

Next, tool forces for a desired tool, work piece, and part program aredetermined in real-time based on measuring the motor power andconverting motor power to tool forces using the transfer functionrelating motor power to tool forces (step 310). However, applicationssuch as NC optimization and integrated tool condition monitoring (TCM)programs require the process model parameters or, in this case, cuttingenergies, so they may look ahead for optimization or compare predictedtool forces or motor power to actual tool forces or motor power for TCM.The predicted tool forces or motor power may be determined based on theprocess parameters determined initially for a sharp tool. Thus, it maybe desirable to determine the process model parameters or cuttingenergies for each tool, work piece, and part program as in the nextstep.

Using the tool forces, a geometric model for the cutting process, andthe process model, the cutting energies (process model parameters) arethen determined (step 312). The user's tool and work piece have processmodel parameters (cutting energies) that differ from the values for thestandard tool and work piece determined in step 302. The measured toolforces from step 310 can be combined with the geometric model of the CNCcutting process to obtain the process model parameters specific for theparticular tool and work piece.

The geometric model of the CNC cutting process provides the geometricinformation used by the process model. The variations in the geometricinformation for the desired part program are obtained using thegeometric model. Normally the variations in the geometry of the firstfew tool cuts will result in a sufficiently rich set of data that willallow the calibration program to perform a best fit of the process modelparameters, thereby calibrating the process model.

If these initial tool cuts in the user's desired part program are notsufficiently diverse to provide a good data fit in determining theprocess model parameters, the tool force measurement and calibrationsystem 46 (FIGS. 2A and 2B) will make a minor modification to the user'spart program. As described below, in the preferred embodiment, theprocess model predicts tool forces based on the material removal rateand the contact area for each tool cut. If the geometry of the initialcuts is such that there is insufficient variation in the materialremoval rate and/or contact area, then the calibration process will nothave sufficient data to carry out a proper best fit. In that case, thetool force measurement and calibration system 46 (FIGS. 2A and 2B) willadjust the feed rate (the speed with which the tool cuts into the workpiece) to a lower than normal value (e.g. to 25% of the programmedfeed). Over a relatively short period of time, typically seconds, thefeed rate is ramped back to the normal value. This variation in feedrate guarantees that the otherwise relatively constant material removalrate varies accordingly. This provides the tool force measurement andcalibration system 46 (FIGS. 2A and 2B) with a sufficiently rich set ofdata to carry out the best fit to the process model parameters orcutting energies.

Using steps 310 and 312, tool forces and process model parameters forany tool, work piece, and part program can be determined by the user atthe user facility in real-time without the use of a force sensor, suchas the force sensor 12 (FIG. 1).

The process model parameters may periodically be updated due to toolwear (step 314). The cutting energies will increase over time as aparticular tool wears. This simply reflects the well-known result thattool forces increase as the tool wears. The variation in cuttingenergies is an important measure of tool wear and is the basis for aprocess-independent tool condition monitoring. The variation in cuttingenergies also affects the NC optimization strategy. For example, if theoptimization condition of interest is maintaining the spindle power ortorque below some maximum allowed value, the NC optimization programrequires the updated values of the cutting energies to properly predictwhat that power or torque will be for the worn tool. Absent this update,the predicted power or torque can be two to three times lower than theactual value, making the NC optimization program relatively useless.

The change in the cutting energies as the tool wears may be determinedby repeating step 312 at suitable time intervals in the CNC partprogram.

It should be noted that the only new requirements of the presentinvention on the end user are to (1) install a non-invasive powersensor, if a suitable open architecture control is not in place, and (2)perform some simple cutting tests with standard tooling and partmaterial. These tests only need to be performed under conditions wherethe transfer function of motor power tool forces is expected to change.This would include an initial set of tests taken over a series of motorloads and rpm. The variation in motor loads would be effected by varyingthe axial depth of cut and/or feed rate. The variation in motor rpmwould be effected by varying the spindle speed and/or the feed rate ofthe feed drive motors. These variable conditions will all be codified inthe standard part program provided to the end user in step 304, so fromthe user's point of view, they merely need to run one simple partprogram for the initial calibration.

The transfer function of motor power to tool forces may be expected tovary from a cool motor (start up) to a warm motor condition. Thus, asdescribed above with respect to steps 306-308, the initial calibrationtests may be run multiple times as the CNC machine warms up to accountfor this modest, but important, trend in the transfer function.

The transfer function of motor power to tool forces may be expected tovary as the CNC changes with usage. For example, spindle or feed motorand coupling degradation may occur due to usage. Thus, as describedabove with respect to step 314, calibration tests should be repeatedperiodically as a maintenance issue, on the order of a few times peryear.

In sum, while the user needs to perform some calibration tests, thosetests will be simple for the user to perform, and may be performed whenthe system is initially installed and at well-spaced maintenanceintervals.

It should be noted that motor power can be consumed internal to themotor of the CNC machine 16 (FIG. 1) or 48 (FIGS. 2A and 2B) due tofriction and other effects, or can be consumed externally in the cuttingprocess. These effects need to be incorporated into the calibrationprocess. Even when there is no cutting, the motor of the CNC machine 16or 48 requires power to continue rotating. The zero load power isreferred to as “tare power.” This tare power needs to be subtracted fromthe measured power to distinguish that portion of total power involvedin the cutting process.

The tare power can change substantially as the motor warms up from thestart-up state. The tare power may also vary with machine rpm andmachine condition. The application of this tool force measurement systemwill require periodic measurements of the zero load power to maintain anaccurate compensation for tare power. The tare power may be determinedat the start of the part program and at each tool change before the toolcuts the work piece and while the tool may safely be considered in anon-cutting condition. This latter condition may be determined using ageometric model of the cutting process, such that periodic updates ofthe tare power may be provided during the cutting process. Except whereotherwise indicated, all references to power contained herein refer toincremental power over tare power.

The process model relates the cutting geometry to the tool forces viathe cutting energies (process model parameters). The simplest processmodel assumes that the spindle motor power is linearly related to thematerial removal rate (the rate at which the tool removes volume as itcuts) as:Ps=K*MRR   [1]where Ps is the spindle power, MRR is the material removal rate and K isthe cutting energy.

For a constant radius tool, the tangential tool force is obtained fromequation [1] as:Ft=K*MRR/V   [2]where Ft is the tangential tool force (the component tangential to thecutting surface of the tool) and V is the tooth speed. V is related tothe spindle speed as (again for a constant radius tool):V=2*pi*R*S   [3]where R is the tool radius and S is the spindle speed.

If the tool does not have a constant radius, then the tool may be splitinto slices along the tool axis and similar relationships derived.

In Equation [1], Ps is an ideal spindle power. In practice, there isalways some residual spindle power required even when the tool is notcutting. This residual power comes from overcoming friction in thesystem and is referred to as the “tare” power. This tare power must besubtracted from the total power.

Similar relationships are available for feed drive power and radialforces. Radial forces are forces perpendicular to the tooth as the toothcuts the material. The total force on a tool in the feed directions inthe cutting plane is a vector sum of the tangential and radial forces oneach tooth.

In sum, the important process model parameters are cutting energies thatdescribe the details of the cutting process. For tangential forcesassociated with the spindle power this parameter is the cutting energyK.

The process model provides a basis for relating spindle and/or feeddrive power to tool forces and to the derivation of the cuttingenergies. Below, two important application areas are described: NCoptimization and tool condition monitoring.

NC Optimization is the optimization of the part program by modifying theprogrammed path and/or technological parameters in order to achieve suchgoals as higher production rates, better surface quality, less wear andtear on the CNC equipment, enhanced tool life, or some combination ofthe preceding. Most frequently, the tool path is not modified such thatthe as-made part remains constant, but the technological parameters suchas spindle speed or feed drive velocity are modified to achieve thedesired objectives.

NC optimization requires an ability to “look ahead” and adjust CNCprocess variables, such as feeds and speeds, to maximize productivityand part quality over the entire cutting process. The look aheadrequires a prediction of the tool forces over the part program. Theoptimization strategy is to modify the part program, such as the feedsand speeds, to adjust tool forces and achieve the optimizationobjective(s). The optimizing feeds and speeds can be selected so as tolimit tool stress, tooth stress, tool vibration, tool/surfacedeflection, and torque/power.

For example, if the objective is to increase productivity, the feeds maybe increased over the programmed values while maintaining tool forceswithin a range providing acceptable surface quality (tool deflection)and also maintaining spindle power and torque values below acceptablemaximum values. Another objective can be increased spindle life or toollife which may be achieved by adjusting the feeds and speeds so that theexcursions (maxima and minima) in the tool forces are alleviated,reducing fatigue and other effects on the spindle and/or tool.

The prediction of the tool forces, both for the original and optimizedpart program, requires geometric and cutting process information, suchas how the tool is and will be engaged with the work piece, coolantconditions, etc., and a reliable process model. Together, the geometricmodel and the process model predict the current and expected toolforces. There are a variety of techniques available for the geometricmodel or virtual CNC. This technology, which is well developed, isreferred to as NC verification. A variety of process models areavailable in the research literature.

The process model, process model parameters, and geometric model(virtual CNC) may then be used with a conventional optimization strategyto determine the tool forces both for current as well as for future toolcuts with the same tool. This anticipated tool force information canthen be used to optimize the cutting process.

Since the measurements are being made in situ, the changes in the toolcondition, such as wear state, are tracked, and this updated informationwill be used to modify the optimization strategy from that suitable foran ideal (sharp) tool.

Tool Condition Monitoring (TCM) is used to monitor the condition of atool in use in a CNC machine. The conditions being monitored may includetool wear, tool or tooth breakage, tool runout (tool offset fromcenterline of the spindle), and/or excessive tool vibrations due to, forexample, chatter.

According to the present invention, the process model parametersdetermined by the method of FIG. 3 may be used along with the processmodel and a geometric model for any desired cutting process to performTCM. More particularly, the process model, process model parameters(cutting energies), and geometric model may be used to predict the toolforces during a cutting process. The actual tool forces may bedetermined in real-time by converting measured motor power to toolforces using the transfer function relating motor power to tool forces,as described with respect to FIGS. 1-3. Then, the actual tool forces maybe compared to the predicted tool forces. Deviations of the actual toolforces from the predicted tool forces indicate conditions such as toolwear or, in more catastrophic cases, tool breakage.

Absolute force values are not necessarily required for tool conditionmonitoring. For example, the time variation of the cutting energies asthe tool wears can be a sensitive indicator of tool wear and impendingsevere tool wear damage. This time variation can be expressed as a ratioof the current cutting energies to the initial (sharp tool) cuttingenergies. By taking a ratio, the motor efficiency (if it is assumedconstant) can be factored out As a result, the calibration process isgreatly simplified, eliminating the need for standard tooling and partmaterials.

The present invention provides substantial opportunity for variationwithout departing from the spirit or scope of the present invention. Forexample, although the above description focuses on sensing motor powerand determining a transfer function for converting motor power to toolforces, the present invention should not be limited thereto. Variousother motor characteristics of the CNC machine 48 may be sensed andthereafter converted to tool forces. For example, in addition to motorpower, motor characteristics that may be sensed include, but are notlimited to, motor torque or current as well as audio, acoustic,ultrasonic, and vibration signals, where a transfer function may bedetermined to convert the measured motor characteristic to tool forcesin a manner similar to that described above for converting motor powerto tool forces.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present invention. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

1. A method for measuring tool forces comprising: determining a forceprofile for a standard tool, work piece, and part program on a firstComputer Numerical Control (CNC) machine based on direct measurement oftool forces; providing the standard tool, work piece, and part programto a user facility; measuring a motor characteristic of a second CNCmachine at the user facility for the standard tool, work piece, and partprogram; determining a relationship between the motor characteristic andtool forces based on the motor characteristic for the standard tool,work piece, and part program and a force profile determined for thestandard tool, work piece, and part program on the first CNC machine;and determining tool forces for a desired tool, work piece, and partprogram on the second CNC machine at the user facility based on themotor characteristic for the desired tool, work piece, and part programand the relationship between the motor characteristic and tool forces.2. The method of claim 1 wherein determining the tool forces for thedesired tool, work piece, and part program comprises: measuring themotor characteristic for the desired tool, work piece, and part program;and converting the motor characteristic for the desired tool, workpiece, and part program to the tool forces for the desired tool, workpiece, and part program using the relationship between the motorcharacteristic and tool forces.
 3. The method of claim 1 whereindetermining the force profile for the standard tool, work piece, andpart program on the first CNC machine based on direct measurement oftool forces comprises determining the force profile for the standardtool, work piece, and part program on the first CNC machine based ondirect measurement of tool forces in a central facility at a firstgeographic location, wherein the user facility is at a second geographiclocation.
 4. The method of claim 1 further comprising repeating thesteps of measuring the motor characteristic of the second CNC machine atthe user facility for the standard tool, work piece, and part programand determining the relationship between the motor characteristic andtool forces for cool and warm motor conditions of the second CNCmachine.
 5. The method of claim 1 further comprising periodicallyrepeating the steps of measuring the motor characteristic of the secondCNC machine at the user facility for the standard tool, work piece, andpart program and determining the relationship between the motorcharacteristic and tool forces such that the relationship between themotor characteristic and tool forces is periodically updated to accountfor wear of the second CNC machine.
 6. The method of claim 1 wherein themotor characteristic is motor power.
 7. The method of claim 1 furthercomprising determining process model parameters based on a process modeland a geometric model and the tool forces for the desired tool, workpiece, and part program.
 8. The method of claim 7 wherein the processmodel parameters comprise cutting energies.
 9. The method of claim 7further comprising predicting tool forces for the desired tool and workpiece based on the process model and the process model parameters. 10.The method of claim 9 further comprising performing Numerical Control(NC) optimization based on the predicted tool forces.
 11. The method ofclaim 9 further comprising performing tool condition monitoring for thedesired tool based on the predicted tool forces.
 12. The method of claim11 wherein performing tool condition monitoring for the desired toolcomprises: measuring the motor characteristic for subsequent cuttingprocesses on the second CNC machine for the desired tool and work piece;converting the motor characteristic for the subsequent cutting processesto tool forces based on the relationship between the motorcharacteristic and tool forces; and monitoring the condition of thedesired tool based on a comparison of the tool forces for the subsequentcutting processes and the predicted tool forces.
 13. The method of claim7 further comprising performing tool condition monitoring for thedesired tool based on the process model parameters.
 14. The method ofclaim 13 wherein performing tool condition monitoring comprises:determining second process model parameters for subsequent cuttingprocesses on the second CNC machine for the desired tool and work piece;and monitoring the condition of the desired tool based on a comparisonof the process model parameters for the desired tool, work piece, andpart program and the second process model parameters for the subsequentcutting processes.
 15. The method of claim 1 wherein determining therelationship between the motor characteristic and tool forces comprisesdetermining a transfer function for converting the motor characteristicto tool forces.
 16. A method for measuring tool forces comprising:receiving a standard tool, work piece, and part program from a centralfacility at a user facility; measuring a motor characteristic of a firstComputer Numerical Control (CNC) machine at the user facility for thestandard tool, work piece, and part program; determining a relationshipbetween the motor characteristic and tool forces based on the motorcharacteristic for the standard tool, work piece, and part program and aforce profile determined on a second CNC machine at a central facilityfor the standard tool, work piece, and part program; and determiningtool forces for a desired tool, work piece, and part program on thefirst CNC machine at the user facility based on the motor characteristicfor the desired tool, work piece, and part program and the relationshipbetween the motor characteristic and tool forces.
 17. The method ofclaim 16 wherein determining the tool forces for the desired tool, workpiece, and part program comprises: measuring the motor characteristicfor the desired tool, work piece, and part program; and converting themotor characteristic for the desired tool, work piece, and part programto the tool forces for the desired tool, work piece, and part programusing the relationship between the motor characteristic and tool forces.18. The method of claim 16 wherein receiving the standard tool, workpiece, and part program further comprises receiving the standard tool,work piece, and part program from the central facility at a firstgeographic location, wherein the user facility is at a second geographiclocation.
 19. A computer readable media comprising software forinstructing a computer to: measure a motor characteristic of a first CNCmachine at a user facility for a standard tool, work piece, and partprogram, the standard tool and work piece provided from a centralfacility; determine a relationship between the motor characteristic andtool forces based on the motor characteristic for the standard tool,work piece, and part program and a force profile determined on a secondCNC machine at the central facility for the standard tool, work piece,and part program; and determine tool forces for a desired tool, workpiece, and part program on the first CNC machine at the user facilitybased on the motor characteristic for the desired tool, work piece, andpart program and the relationship between the motor characteristic andtool forces.
 20. The computer readable media of claim 19 wherein todetermine tool forces for the desired tool, work piece, and partprogram, the computer readable media further instructs the computer to:obtain measurements of the motor characteristic for the desired tool,work piece, and part program; and convert the motor characteristic forthe desired tool, work piece, and part program to the tool forces forthe desired tool, work piece, and part program using the relationshipbetween the motor characteristic and tool forces.